This invention relates generally to refrigeration and air conditioner operation and more particularly to a method and apparatus for boosting the cooling capacity and efficiency of refrigeration and air-conditioning systems under a wide range of ambient atmospheric conditions.
In air conditioning, the basic circuit is essentially the same as in many refrigeration systems. It comprises an evaporator, a condenser, an expansion valve or a float valve, and a compressor. This, however, is where the similarity ends. The evaporator and condenser of an air conditioner will generally have less surface area. The temperature difference .DELTA.T between condensing temperature and ambient temperature is usually 27.degree. F. with a 105.degree. F. minimum condensing temperature, while in refrigeration the difference .DELTA.T can be from 8.degree. F. to 15.degree. F. with an 86.degree. F. minimum condensing temperature. Another difference exists in the types of compressors and refrigerants that are used. Smaller systems typically use positive displacement-type compressors with capacities up to about 3,000 intake cubic feet per minute (ICFM). Larger systems use centrifugal-type compressors, with capacities from a few thousand to over 100,000 ICFM. See "Compressor Selection for the Chemical Process Industries," Chemical Engineering, pp. 78-94, Jan. 20, 1975. Centrifugal compressor-type systems typically operate at lower system pressures than positive displacement-type systems.
I have previously improved the cooling capacity and efficiency of refrigeration systems of the type that use a positive displacement compressor and expansion valve. As disclosed in my U.S. Pat. No. 4,599,873, this is accomplished by addition of a centrifugal liquid pump at the outlet of the receiver or condenser. Operation of the pump typically adds 5-12 p.s.i. of pressure to the condensed refrigerant flowing into the expansion valve, a process I call liquid pressure amplification. This increment of pressure added by the centrifugal pump suppresses flash gas and assures a uniform flow of liquid refrigerant to the expansion valve, substantially increasing cooling capacity and efficiency. The best results are obtained in a floating head system operated with the condenser at moderate ambient temperatures, usually under 80.degree. F. As ambient temperatures rise above the minimum condensing temperature, the advantages gradually decrease. The same thing happens when the principles of my prior invention are applied to air conditioning, except that the minimum condensing temperature is higher.
My prior U.S. Pat. No. 5,180,580 discloses a way to further improve upon the U.S. Pat. No. 4,599,873. By having the liquid pump feed a fraction of the pressure-augmented refrigerant to the inlet of the condenser, that invention extends the advantages of liquid pressure amplification to higher ambient temperature conditions. This specification includes the detailed description of this improvement with reference to FIGS. 3-5. I had thought that the basic principles of this patent could be readily extended to centrifugal compressor-type systems, by positioning the liquid refrigerant pump in a branch line from the condenser outlet to the condenser inlet, but that proved not to be practical.
Holtzapple U.S. Pat. No. 5,097,677 describes a method for refrigeration and air conditioning whereby refrigerant vapors are desuperheated by recycling liquid refrigerant from the condenser outlet back to the compressor outlet. In one embodiment (FIGS. 6 and 7), Holtzapple discloses spray injection of liquid refrigerant into the centrifugally-compressed refrigerant gas stream to suppress superheat, but does not identify the means by which the liquid refrigerant is input to the spray injector. FIG. 8a of the Holtzapple patent illustrates recycling of a portion of the liquid refrigerant back to the outlet of a reciprocating or positive displacement compressor using what Holtzapple calls a conventional hydraulic pump.
Holtzapple projects an improvement in refrigeration performance of these and other systems in a series of three theoretical papers published in ASHRAE Transactions 1989, V.95, Pt. 1, entitled "Reducing Energy Costs in Vapor-Compression Refrigeration and Air Conditioning Using Liquid Recycle--Part I: Comparison of Ammonia and R-12" (No. 3221); "Reducing Energy Costs in Vapor-Compression Refrigeration and Air Conditioning Using Liquid Recycle--Part II: Performance" (No. 3222); and "Reducing Energy Costs in Vapor-Compression Refrigeration and Air Conditioning Using Liquid Recycle--Part III: Comparison to other Energy-Saving Cycles" (No. 3223). Based on my own research and development mentioned above, the Holtzapple patent and papers overlook certain practical aspects that seriously impact the ability to apply liquid recycle to real systems, especially in centrifugal compressor-type systems.
A major problem with drawing liquid refrigerant from the condenser outlet is that the refrigerant tends to flash or vaporize. In low pressure systems, when the condensed refrigerant exits the condenser, the pressure tends to decrease proportionately more quickly than its temperature. This pressure decrease can be substantial in a centrifugal compressor-type refrigeration system having a high-low float valve, rather than an expansion orifice, to meter refrigerant into the evaporator. Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating centrifugal pump, such as used for liquid pressure amplification, the vapor bubbles are carried along to a point of higher pressure where they suddenly collapse. This phenomenon is known as cavitation. Cavitation is a serious problem because it results in metal removal, vibration, reduced flow, loss in efficiency, and noise. In addition, cavitation may occur in the pump inlet which can damage the impeller vanes near the inlet edges.
When employing centrifugal liquid pumps, certain precautions must be taken to avoid cavitation and the problems it causes. At the centrifugal pump inlet, it is necessary to maintain a required net positive suction head (NPSH).sub.R which is the equivalent total head of liquid at the pump centerline less the vapor pressure of the refrigerant. To avoid cavitation, the available net positive suction head (NPSH).sub.A must be equal to or greater than the (NPSH).sub.R. The (NPSH).sub.A depends on the total head, the pump speed, the capacity, and impeller design.
More importantly, in trying to pump liquid refrigerant the (NPSH).sub.A depends on the characteristics of the refrigerant, such as its vapor pressure and its temperature and pressure when it enters the inlet of the centrifugal pump. For example, refrigerant R-11 will flash readily. Moreover, centrifugal compressor systems, which most commonly use R-11, typically operate in a pressure range in which flashing can easily occur. Such a system typically has an evaporator pressure of about half an atmosphere and a condenser pressure of one to two atmospheres. Also, these pressures can fluctuate substantially as the float valve opens and closes to meter liquid refrigerant into the evaporator, causing refrigerant to be drawn away from the liquid pump inlet and reducing NPSH.
This fluctuation is not a problem in the system described in my prior U.S. Pat. No. 5,180,580. All of the refrigerant discharged from the condenser is input to the centrifugal liquid pump's intake, and so the intake pressure is maintained at a consistently high level. Also, the use of a positive displacement-type compressor will ordinarily maintain a higher pressure at the centrifugal liquid pump's intake. This arrangement would be impractical in a centrifugal compressor-type system, however, because adding an increment of pressure to entire liquid flow from the condenser would increase the pressure of the liquid refrigerant against a closed float valve sufficiently to overfeed the liquid recycle branch to the condenser. As discussed by Holtzapple, this is undesirable.
Moreover, in recent years there has been a growing concern that chlorofluorocarbon (CFC) refrigerants, such as R-11, tend to damage the earth's ozone layer. As a result, state and federal legislation are being enacted which significantly regulates the use of such refrigerants. Thus, more and more, manufacturers of refrigerant systems are developing alternatives to CFC refrigerants. The suggested alternatives, such as blends of non-CFC refrigerants, although more environmentally sound, have lower vapor pressures and therefore flash more easily. Accordingly, conventional refrigeration systems which use the new environmentally desirable refrigerants are even more susceptible to flashing and therefore limit the use of conventional centrifugal compressor-type systems even further.
Holtzapple's approach is to employ a conventional hydraulic pump to recycle a portion of the liquid refrigerant to the condenser inlet. Common hydraulic pumps, such as rotary vane pumps and gear pumps, are positive displacement pumps that do not require a net positive suction head to operate. Thus, low vapor-pressure refrigerants, which easily flash and cause cavitation in centrifugal pumps, can be readily pumped by such hydraulic pumps. Hydraulic pumps have several drawbacks, however, when employed in refrigeration systems. They have metal to metal contact which makes them very susceptible to wear. Thus, they must be continuously lubricated to be operational. Preferably, hydraulic pumps are lubricated by the fluid they are pumping, such as oil or water. Refrigerants such as R-11 are, however, actually solvents that provide little or no lubricating effect. This factor also makes it impractical to use oil-type lubricant, as they will be dissolved and contaminate the refrigerant. As a result, conventional hydraulic pumps, used continuously in refrigerant systems, would rapidly wear, require constant maintenance, and prematurely fail. Thus, in Holtzapple's system, any efficiencies gained by desuperheating using recycled refrigerant would be offset by the inefficiencies associated with using a hydraulic pump.
Centrifugal pumps, on the other hand, do not need lubrication and are now the standard pump used in refrigeration systems. Further advantages of centrifugal pumps are their simplicity, low first cost, uniform (nonpulsating) flow, small floor space, low maintenance expense, quiet operation, and adaptability for use with a motor. Thus, it is preferable to utilize centrifugal pumps whenever design parameters permit. In centrifugal compressor-type systems, however, this has proven to be unsatisfactory because of the NPSH-cavitation problems. Also, it is not practical to utilize floating head pressures in such a system, as in a positive displacement-type system, because of the need to use anti-surge controls in centrifugal compressor-type systems.
Accordingly, a need remains for a refrigeration or air-conditioning system which combines the benefit of desuperheating using liquid refrigerant recycle with the practicalities and efficiencies of centrifugal liquid pumping means.